What will a glass look like in 500 years?

Question

The glass is in a metastable state. It is changing constantly. So what will a piece of glass look like in 500 years in room temperature?

Answer 1

It's a common misconception that glass flows appreciably over time. So the answer is that your glass will have exactly the same shape as it does now, to within under a wavelength.

You can debunk the myth of flowing glass with simple experiment / observation. There are on Earth several very large refracting telescopes over 100 years old with huge lenses ground to precise optical specifications. Astronomy, as with microscopy or any precision imaging, is an application that is extremely sensitive to optical aberrations, and even sub-wavelength deformations in the refractors of these telescopes over time would degrade the telescopes' optical performances. Yet even these sensitive devices have suffered no plastic deformation that would shift their surfaces more than a significant fraction of a wavelength, which they would do over 100 years if glass were in the state of flow that is commonly claimed.

You can make this simple observation quantitative if you can borrow of the order of a week's time on a decent interferometer. Set up a large, low cost lens (say of the order of 0.1m diameter or more) to focus into the focus of a spherical retroreflector and light the system with the probe beam of a Fizeau interferometer. Cement a heavy piece of steel or lead to the top of the lens (you'll need to do this so that you can still pass an appreciable beam from the interferometer through the unobscured part of the lens) and record the fringe pattern. You'll find that after a week, the fringe pattern has not shifted AT ALL. You can then derive an upper bound to the amount of "flow" that the lens has undergone from this observation. If the top surface hasn't moved 20 nanometers (which shift would be highly observable on the interferometer) in a week (the result I can assure you you will observe), its motion is then less one micron every year, i.e. less than than 1 millimeter deformation every thousand years!